implemented configuration file validation, closes #11
added nose tests to check if configuration validation works as expected
oops, includes are optional!
mandatory sections should only be checked once and not once for each section!
configuration moved to own module, throws own Exception "InvalidConfig"
improved configuration parser to allow for non-trivial secton names
added Validator for Shortcut section, added test for Shortcut section
simplified config reader, moved import of configreader into main
entry_point in Shortcut is not mandatory
renamed invalid_config_keys.cfg to invalid_config_section.cfg
made entry_point in Application optional
renamed all ocurrences of subsections to keys
Merge pull request #13 from devsnd/master

implemented configuration file validation and added tests for them, closes #11